The principal objectives of this study are: a. To evaluate the relationship between specific mammographic parenchymal characteristics and the incidence of female breast cancer; b. to evaluate the relationship between mammographic parenchymal characteristics and risk factors for breast cancer. Other objectives are to identify histologic correlates of mammographic parenchymal characteristics and to evaluate both right-left concordance of parenchymal pattern, and observer reliability in the assessment of these patterns. All women (including 800 new cases of breast cancer) having xeromammograms in any of seven collaborating radiology units during 1978 or 1979 will be enrolled. Information on status with respect to breast cancer risk factors, parenchymal pattern (Wolfe's classification) and pathologic diagnosis will be obtained from the subjects' radiology records. From this series, 400 cases and 800 controls, equally divided between the age groups less than 55 and 55-74, will be selected for a detailed "blind" review of both xeromammogram and histologic slides. For women in each age group relative risk of breast cancer according to parenchymal pattern, with the N1 pattern as the referent, will be estimated. If necessary, these estimates will be corrected for confounding by other risk indicators. Possible interactions will also be quantified.